The present invention relates to a protective cover for a window or a window insert which is IR transparent for use in conjunction with infrared devices and particularly for utilization in equipment which will respond to IR bands within the so-called third atmospheric window extending from 8 to 13.5 micrometer, the cover or window itself is to serve as protection against rain-induced erosion or other moisture. The invention will particularly refer to covers in apparatus and devices to be used in aircraft or ships, whereby the latter are particularly in need of protection against moisture and salt water spray in general.
Infrared sensors are quite useful and have significant importance in the area of military reconnaissance, target acquisition, etc. under utilization of infrared radiation. Herein the so-called third atmospheric "window" in the large wave infrared range from 8 to 13.5 micrometer has gained increasing importance because in this particular range of wave length, objects having generally a temperature which does not differ significantly from the environmental temperature can in fact be made "visible" through the thermal radiation emission, having a significant component within this particular band and "window".
Infrared systems arranged to look ahead, i.e., in forward direction, and being mounted in aircraft or the like, or infrared systems in ships such as infrared periscopes in submarine vehicles or night-sight equipment of regular vessels, all have to be protected against the environment in one form or another. The protection specifically has to work against rain and erosion due to rain or against sea water. Usually this protection is provided through a suitable cover producing IR windows, and sometimes being configured as an IR dome. Here, however, it has to be considered that most solid objects are opaque with respect to radiation in the range of interests from 8 to 13.5 micrometer, so that only very few materials are suitable at all from the point of view of infrared transmission. This of course does not mean that any such material is also suitable for a protective purpose as far as rain and other moisture is concerned.
Considering the matter in some detail it is, for example, well known that germanium zinc-sulphide, gallium arsenide and zinc selenide are indeed transparent to this long wave infrared radiation. However, none of these materials can be regarded as erosion proof when exposed to rain. Thus, any all-weather utilization is practically excluded as far as these materials are concerned. This is particularly noticeable with regard to zinc selenide, which material is well known for its very low transmission and absorption losses in the long range of IR wave, particularly for waves in the range from 8 to 13.5 micrometer. Zinc selenide will exhibit a 50% IR transmission loss at a speed of the equipment of about 0.68 mach and a rain intensity of 25 millimeter per hour within about 10 seconds.
Polyurethane covers are used, for example, for protecting glass fiber reinforced synthetics, because such a material is considerably more resistant against rain erosion. However, polyurethane is not transparent for infrared and is, therefore, unsuitable for the intended task right from the beginning. Silicon on the other hand is also a well proven infrared window material in the middle infrared range, and the period of time in which transmission will be reduced to about 50% is about a thousand times longer than in the case of zinc selenide (i.e., about 3 hours). However, silicon is not a suitable window material from the point of view of transmission in the long infrared range. Thus, again this material is not suitable for the intended task.
As far as is known, materials which are in fact known to have infrared transmission in the long wave range simply exhibit consistently very poor resistance against rain erosion, and are therefore not suitable, for example, in aircraft. Even relatively slow flying aircraft still expose such material to the rain erosion to such an extent that loss in transmission occurs rapidly.